This invention generally relates to interferometry and more particularly to apparatus and methods by which aspheric surfaces and wavefronts may be rapidly and precisely measured.
Aspheric surfaces offer significant advantages to the designers of high performance optical systems, but their widespread adoption has been hampered by difficulties in manufacture and measurement. Next generation lithography systems, however, are being designed with aspheric surfaces with apertures approaching 0.5 meters, high numerical apertures, and tolerances on the surface shape of less than 1 nm. A large number of approaches to the measurement of aspheric surfaces and wavefronts have been described over the years, each with advantages and disadvantages.
Kxc3xcchel, in U.S. Provisional Patent Application No. 60/299,614 filed on Jun. 20, 2001 (now U.S. patent application Ser. No. 10/160,672 filed on Jun. 3, 2002 and published on Jan. 2, 2003 as US-2003-0002049 A1), introduced new approaches to measuring aspheric optics in a scanning interferometer. This was extended in U.S. Provisional Patent Application No. 60/303,856 filed on Jul. 9, 2001 (now U.S. patent application Ser. No. 10/180,286 filed on Jun. 26, 2002 and published on Mar. 6, 2003 as US-2003-0043385 A1) to allow explicitly for the collection of the data (to be reduced in the manner taught in the 60/299,614 application, id.) using a Fizeau interferometer. Such scanning approaches described by Kxc3xcchel and others (e.g. Tronolone, U.S. Pat. No. 5,416,586) tend to be slow, leading Zanoni in U.S. Provisional Patent Application No. 60/299,512 filed on Jun. 20, 2001 (now U.S. patent application Ser. No. 10/152,075 filed on May 21, 2002 and published on Jan. 2, 2003 as US-20030002048 A1) to propose the use of an interferometer with an aspheric reference surface which would be calibrated elsewhere, for example, following the method of Kxc3xcchel. Measuring the reference surface on a separate instrument introduces issues of stability in the transfer from one instrument to another, as well as practical issues relating to the frequency of recalibration. These issues were addressed by Evans and Kxc3xcchel in U.S. Provisional Patent Application No. 60/317,028 filed on Sep. 4, 2001 (now U.S. patent application Ser. No. 10/233,772 filed on Sep. 3, 2002 and published on March 13, 2003 as US-2003-0048457 A1), who proposed an approach in which the aspheric reference surface of a Fizeau interferometer is mastered by scanning with a reference sphere. The aspheric part to be measured is then inserted into the instrument (without dismounting the reference asphere). The approach of the 60/317,028 (10/233,772, id.) Application presumes that a reference sphere of sufficient, known quality is available and can be installed in the instrument without distortion.
Accordingly, It is the object of this invention to provide a measurement process which provides for traceable calibration of aspheric optics.
It is a further object of the invention to provide a procedure for continuous improvement of the uncertainty of the measurement made using interferometric instruments.
It is another object of this invention to provide a process with inherent quality assurance, allowing monitoring of the stability of the measurement process.
Other objects of the invention will, in part, appear obvious and will, in part, appear hereinafter when the following detailed description is read in connection with the accompanying drawings.
This invention provides apparatus and methodology for traceable measurement of aspheric optical surfaces or wavefronts with internal self-consistency checks through two parallel routes to part measurement. Apparatus comprises a Fizeau interferometer which can be configured with optics that produce either spherical or deliberately aspheric wavefronts and with means of scanning test surfaces along the optical axis. Algorithms and procedures requiring multiple measurements are provided to compensate measurement data for bias arising from error motions of the slideway. Provision is made for storing and frequently renewing instrument calibration data. Trend analysis of that frequently retaken calibration data is used to indicate and, hence, manage measurement system performance.